1. Field of the Invention
The present invention relates to a spin cleaning method of cleaning a substrate to be treated, such as a semiconductor wafer or an LCD substrate.
2. Description of the Related Art
A process for manufacturing a semiconductor device or a liquid crystal display device (LCD) generally includes a step of cleaning a substrate to be treated, such as a semiconductor wafer or an LCD substrate. The cleaning step is intended to remove particles adhered to the surface of the substrate and a natural oxide film which has been formed by exposing the substrate to the atmosphere. As an apparatus for carrying out the cleaning step, there are a batch type in which a lot of substrates are treated in a process chamber at the same time and a singular type in which substrates are treated in a process chamber one by one. A cleaning apparatus of the singular type generally. utilizes a spin cleaning method.
In the spin cleaning method, a substrate to be treated is held and rotated by a spin chuck. The surface of the substrate is supplied with a chemical liquid and then with pure water used as a rinsing liquid, and is sequentially subjected to a spin drying operation. In addition to make the pure water spun off by a centrifugal force, an inactive gas or clean air may be sprayed onto the surface of the substrate to facilitate the spin drying operation, when the substrate is dried.
A trace of the rinsing liquid, so called "water mark", left on the substrate when dried, is used to evaluate a cleaning state of the substrate. FIGS. 12A to 12D are views schematically showing a manner of a water mark being formed on the surface of, e.g, a semiconductor wafer W, when the surface of the wafer is treated with hydrofluoric acid. At first, as shown in FIG. 12A, a hydrofluoric acid solution 15 is supplied onto the surface of the wafer W while the wafer W is held and rotated by a chuck 11. Then, as shown in FIG. 12B, pure water 16 is supplied from a nozzle 13 to rinse the surface of the wafer W while the pure water is spun off by a centrifugal force. During this step, part of the pure water is left on the surface of the wafer, as shown in FIG. 12C. As a result, a water mark 14 is formed to correspond to the left part of the pure water 16, as shown in FIG. 12D.
It is thought that the water mark is generated as follows. That is, a very small amount of silica (SiO.sub.2) is originally contained in the pure water. Further, H.sub.2 SiO.sub.3 is produced by reaction among water, oxygen in air, and silicon of the wafer, as shown in reaction formula (1). EQU Si+H.sub.2 O+O.sub.2 .fwdarw.H.sub.2 SiO.sub.3 ( 1)
As a result, a very small amount of SiO.sub.2 and H.sub.2 SiO.sub.3 is contained in the pure water used as the rinsing liquid. On the other hand, while the pure water is being dried on the surface of the wafer, the pure water is divided into a plurality of portions and contracted due to its surface tension. The impurities, SiO.sub.2 and H.sub.2 SiO.sub.3, contained in the pure water are gathered with the pure water being dried, and precipitated as particles on the surface of the wafer, thereby forming water marks.
Especially in the hydrofluoric acid treatment, SiO.sub.2 on the surface of the wafer W is removed and Si is exposed, so that the reaction as shown in the reaction formula (1) easily occurs. Further, where a hydrophobic film 18, such as polysilicon, is deposited and a recess 19 is formed on the surface of the wafer W, hardly any of the pure water is spun off so that water marks are apt to be left.
As described above, water marks are formed of impurity particles. With a decrease in the line width of a semiconductor device, a more strict administration is required in relation to particle contamination. As a result, it has been deemed necessary to prevent water marks from being formed.
Jpn. Pat. Appln. KOKAI Publication No. 5-243205 discloses a method of removing a liquid from a substrate surface in a centrifugal machine. In this method, an organic solvent vapor is supplied onto the substrate, which is held by a spin chuck with a vacuum force and is preferably non-rotated, so that the liquid on the substrate is contracted by means of the Marangoni effect. Then the substrate is rotated so that the liquid is removed from the substrate. The organic solvent vapor is supplied in a non-saturated state and at a temperature lower than that of the substrate, so that the organic solvent is prevented from being condensed on the substrate.
In this method, however, it is necessary to make improvements in terms of temperature administration of the organic solvent vapor and safety. Further, there is a problem in that the organic solvent vapor is hardly diffused in a uniform state. Still further, where a so-called spin chuck of the mechanical type, in which a wafer is fixed to the chuck by clamp fingers, is used, water marks are possibly formed at the portions engaged with the clamp fingers.